Broadband dielectric reflectors for LED with varying thickness

ABSTRACT

A broadband, omnidirectional, multi-layer, dielectric reflector for an LED in a white light emitting device provides both near 100% reflectivity across the visible spectrum of light, and electrical insulation between the substrate and the electrical circuitry used to power and control the LED. When a sealant material, having a higher index of refraction than air, is used to protect the LED and the accompanying electrical circuitry, an aluminum reflector layer or substrate is provided to make up for the loss of reflectivity at certain angles of incidence. The dielectric reflector includes two separate sections with two different thicknesses, a thinner section below the LED providing better heat conductivity, and a thicker section surrounding the LED providing better reflectivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation in part of U.S. patentapplication Ser. No. 13/437,742, filed Apr. 2, 2012, which isincorporated herein by reference. The present invention also claimspriority from U.S. patent application Ser. No. 61/720,199 filed Oct. 30,2012, which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an all-dielectric reflector for a lightemitting diode (LED), and in particular to a broadband, omnidirectional,multi-layer, all-dielectric reflector with varying thickness for an LEDin a white light emitting device providing reflectivity, heatconductivity, and electrical insulation.

BACKGROUND OF THE INVENTION

Conventional white light emitting devices, such as those disclosed inUnited States Patent Application 2011/0186874 published Aug. 4, 2011 inthe name of Shum and illustrated in FIG. 1, include a blue or UVemitting LED 1 mounted on or surrounded by an electric circuit 2, whichprovides electrical connections to an outside power source to power theLED 1. A metallic reflective layer 3, e.g. silver, is disposed on asilicon substrate 4 for reflecting any stray light refracted orreflected by the LED package back in the desired direction. Accordingly,an isolation layer 6 is also required to provide electrical insulationbetween the electric circuit 2 and the metallic reflector layer 3.Nitrides or oxides, such as Si₃N₄ or SiO₂, are often used for theisolation layer 6 resulting in thicknesses of approximately 2 μm to 6μm, depending on the legal requirements for the breakthrough voltages.Typically, the LED 1 is immersed in a light transmitting epoxy 7, whichincludes light converting dyes, e.g. phosphor, for converting the lightemitted from the LED into a broad spectrum white light. Unfortunately,the silver reflective layer 3 is not environmentally stable over timeand becomes tarnished, especially when exposed to high temperature andhumidity, which greatly reduces the effective lifetime of the lightemitting device.

The use of narrowband dielectric reflectors to reflect the particularwavelength of light emitted by the LED back through the phosphormaterial to maximize wavelength conversion has been disclosed in U.S.Pat. No. 6,833,565 issued Dec. 21, 2004 to Su et al; however, theproblems of broadband, white light absorption/reflection and electricalisolation are not addressed.

An object of the present invention is to overcome the shortcomings ofthe prior art by providing a white-light emitting LED device in which abroadband, omnidirectional, multi-layer, dielectric reflector withvarying thickness is used for providing reflectivity, thermalconductivity, and electrical insulation.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a light emitting diode(LED) device comprising:

a substrate;

an LED for emitting light at a first wavelength;

electrical circuitry for providing power to the LED from externalsources;

a wavelength conversion material covering the LED for converting lightemitted at the first wavelength to light of at least a secondwavelength, which combined with the light of the first wavelength formsa broadband light source; and

a multi-layer dielectric structure of alternating high and low indexmaterial layers in between the substrate and the LED providing bothelectrical insulation for the electrical connections and reflectivityfor the broadband light source;

wherein the multi-layer dielectric structure has a first area below theLED with a first thickness providing electrical isolation and heattransfer to the substrate, and a second area with a second thicknessthicker than the first thickness surrounding the LED providingelectrical isolation and reflectivity for the light.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1 is a cross-sectional view of a convention LED device;

FIG. 2 is a cross-sectional view of an LED device according to thepresent invention;

FIG. 3 is a plot of reflectance vs wavelength with a zero angle ofincidence for four wavelength stacks having center wavelengths spaced100 nm apart;

FIG. 4 is a plot of reflectance vs wavelength with a zero angle ofincidence for a combined wavelength stack according to a firstembodiment of the present invention including four sections havingreflectance bandwidths defined by center wavelengths spaced 100 nmapart;

FIG. 5 is a plot of reflectance vs wavelength with a zero angle ofincidence for a combined wavelength stack according to a secondembodiment of the present invention including approximately fifty highand low index pairs having reflectance bandwidths defined by centerwavelengths spaced 10 nm apart;

FIG. 6 a is a plot of reflectance vs wavelength for the wavelength stackof the second embodiment at a plurality of different angles ofincidence;

FIG. 6 b is a plot of reflectance vs wavelength for an optimizedwavelength stack of the second embodiment at a plurality of differentangles of incidence;

FIG. 7 is a plot of average reflectance over a range of 460 to 680 nm vsangle of incidence comparing metallic reflectors, e.g. silver andaluminum, to the all dielectric reflectors in accordance with thepresent invention;

FIG. 8 is a plot of average reflectance over a range of angles ofincidence vs wavelength comparing metallic reflectors, e.g. silver andaluminum, to the all dielectric reflectors in accordance with thepresent invention;

FIG. 9 is a cross-section view of an LED device in accordance with analternative embodiment of the present invention;

FIG. 10 is a cross-sectional view of an LED device in accordance with analternative embodiment of the present invention;

FIG. 11 is a plot of average reflectance over a range of 460 nm to 680nm vs angle of incidence comparing metallic reflectors, e.g. silver, tothe all dielectric reflector in accordance with the second embodiment ofpresent invention, and the combination of the all dielectric reflectorin accordance with the second embodiment of the present invention withan aluminum reflector layer;

FIG. 12 is a plot of average reflectance over a range of angles ofincidence vs wavelength comparing metallic reflectors, e.g. silver, tothe all dielectric reflector in accordance with the second embodiment ofpresent invention, and the combination of the all dielectric reflectorin accordance with the second embodiment of the present invention withan aluminum reflector layer;

FIG. 13 a is a plot of reflectance over a range of angles of incidencevs wavelength;

FIG. 13 b is a plot of average reflectance vs angle of incidence for therange of wavelengths between 400 nm and 700 nm; and

FIG. 14 is a cross-section view of an LED device in accordance with analternative embodiment of the present invention.

DETAILED DESCRIPTION

With reference to FIG. 2, the present invention relates to a broadband,e. g. white, light emitting diode (LED) device, generally indicated at11, including a substrate 12, typically formed of silicon or some othersemiconductor material, glass (e.g. silica) or in a preferredembodiment, a metallic broadband reflective material, e.g. aluminum. Oneor more LED chips 13, e.g. a blue LED made of InGaN, are mounted on thesubstrate 12 for generating light in a first narrowband wavelengthrange, e.g. 50 nm, typically in the blue or ultra-violet (UV) range.Electronic circuitry 14, preferably in the form of conductive tracespatterned in a conductive oxide layer, e.g. indium tin oxide, is formedaround the LED 13 and electrically connected to external power andcontrol sources to provide power and other control and monitoringfunctions to the LED 13.

In a preferred embodiment, the LED 13 and the electronic circuitry 14are encapsulated in a transparent sealant material 16, e.g. silicone,which protects the LED 13 and the electronic circuitry 14. Additionally,the sealant material provides insulation and sealing for the electricalcontacts and wires making up the electronic circuitry 14. Other types ofmaterial can be used, such as epoxy, glass, spin on glass, plastic,polymer, metal or semiconductor material.

In a preferred embodiment, the sealant material 16 includes polymers,which begin in a fluidic state for filling and sealing an interiorregion around the LED 13. The sealant material 16 is then cured to forma substantially stable state. The sealant material 16 is preferablyoptically transparent or can also be selectively transparent and/ortranslucent according to specific requirements. In addition, the sealantmaterial 16, once cured, is substantially inert, and has a lowabsorption capability to allow a substantial portion of theelectromagnetic radiation generated by the LED 11 to traversetherethrough.

The generation of white light involves passing the light at the initialwavelength, shown as a solid line in FIG. 2, from the LED(s) 11 througha medium including one or more different wavelength conversionmaterials, e.g. phosphors, creating different wavelengths, e.g. colors,of light, shown as dashed lines in FIG. 2, which combine to form whitelight. In a preferred embodiment, wavelength conversion is provided bymaterials that convert electromagnetic radiation absorbed by thewavelength conversion material. In a specific embodiment, the wavelengthconversion material 16 is excited by the primary emission of the LED 13and emits electromagnetic radiation of at least a second wavelength andoptionally third and fourth wavelengths, as well.

In a preferred embodiment, the wavelength conversion material isincorporated into the sealant material 16, thereby surrounding andcovering the LED 11 on five sides. In use, a fraction of the light fromthe LED 11 undergoes the Stokes shift, and is transformed from shorterwavelengths to longer wavelengths, and if several different phosphormaterials of distinct colors are provided within the sealant material16, the emitted spectrum is broadened even more, preferably to over 150nm, more preferably over 200 nm, and even more preferably over 250 nm,effectively raising the color rendering index (CRI) value of a givenLED.

As an example: a phosphor-based white LED device 11 according to thepresent invention includes an encapsulate InGaN blue LED 13 inside aphosphor coated epoxy 16, in which the phosphor is a common yellowphosphor material, such as cerium-doped yttrium aluminum garnet(Ce3+:YAG). Some of the original blue light (420 nm-490 nm) combineswith the transformed yellow light (550 nm-600 nm) to appear as whitelight.

White LED devices 11 can also be formed using a near-ultraviolet (NUV)LED 11, generating UV light in the 350 nm to 400 nm range, with asealant material 16 incorporating a mixture of high-efficiencyeuropium-based phosphors that emit red and blue light, along with copperand aluminum-doped zinc sulfide (ZnS:Cu, Al) that emits green light. Thegenerated red (620 nm to 700 nm), green (490 nm to 570 nm) and blue (420nm to 490 nm) light combine to form a white light, as is well known inthe art.

In other embodiments, the sealant material 16 can be doped or treatedwith other wavelength adjusting materials to selectively filter,disperse, or influence the original wavelength of light into one or morenew wavelengths of light. As an example, the sealant material 16 can betreated with metals, metal oxides, dielectrics, or semiconductormaterials, and/or combinations of these materials to convert theoriginal wavelength to one or more different wavelengths of light with amuch wider overall bandwidth, e.g. 150 nm or more, generating whitelight.

In alternate embodiments, phosphor particles may be deposited onto theLED 13 or the LED device 11. The phosphor particles may comprise any ofthe wavelength conversion materials listed above, or other materialsknown in the art. Typically, the phosphor particles may have amean-grain-diameter particle size distribution between about 0.1 micronand about 50 microns. In some embodiments, the particle sizedistribution of phosphor particles is monomodal, with a peak at aneffective diameter between about 0.5 microns and about 40 microns. Inother embodiments, the particle size distribution of phosphor particlesis bimodal, with local peaks at two diameters, trimodal, with localpeaks at three diameters, or multimodal, with local peaks at four ormore effective diameters.

In a specific embodiment, the entities comprises a phosphor or phosphorblend selected from (Y,Gd,Tb,Sc,Lu,La)₃(Al,Ga,In)₅O₁₂:Ce³⁺,SrGa₂S₄:Eu²⁺, SrS:Eu²⁺, and colloidal quantum dot thin films comprisingCdTe, ZnS, ZnSe, ZnTe, CdSe, or CdTe. In other embodiments, the devicemay include phosphors capable of emitting substantially red and/or greenlight. In various embodiments, an amount of phosphor material isselected based on the color balance of the blue LED devices.

To replace both the isolation layer 6 and the metallic reflector layer3, the present invention utilizes a multi-layer dielectric stack 17 ofalternating high and low index layers 18 and 19, respectively, designedto provide a broadband reflector, e.g. greater than 150 nm, preferablygreater than 200 nm, and more preferably greater than 250 nm, andelectrical isolation between the electronic circuitry 14 and thesubstrate 12. Ideally, the dielectric stack 17 reflects at least 90% ofthe light in the visible spectrum, i.e. 390 nm to 720 nm (330 nm wide);however, somewhat smaller bandwidths are within the scope of theinvention, e.g. 400 nm to 680 nm (280 nm wide) and 440 nm to 640 nm (200nm wide).

Typically, the low index layers 19 are comprised of SiO₂, while the highindex layers 18 are comprised of one or more of Ta₂O₅, Nb₂O₅, and TiO₂;however, other materials for both the high and low index layers 18 and19 are within the scope of the invention.

In a first embodiment, the dielectric stack 17 is comprised of 4 or 5sections, each section adapted to reflect light in a predeterminedwavelength range, e.g. 75 nm to 125 nm, defined by a center wavelength,

_(C), which are spaced apart from other center wavelengths byapproximately the same distance, e.g. 75 nm to 125 nm, so that there isa small overlap in bandwidth. Accordingly, each section is comprised of8 to 20 layers of alternating high and low index material having athickness approximately equal to a quarter wavelength of the centerwavelength of the predetermined range.

For example: the first section of the dielectric stack 17 is comprisedof 8 to 20 layers of alternating high and low index material, each layerhaving an optical thickness of

/4 of

_(C1) (450 nm/4)=112.5 nm. For actual thickness divide the opticalthickness by the refractive indices at these wavelengths.

The second section of the dielectric stack 17 is comprised of 8 to 20layers of alternating high and low index material, each layer having anoptical thickness of

/4 of

_(C2)=(550 nm/4)=137.5 nm.

The third section of the dielectric stack 17 is comprised of 8 to 20layers of alternating high and low index material, each layer having anoptical thickness of

/4 of

_(C3)=(650 nm/4)=162.5 nm.

The fourth section of the dielectric stack 17 is comprised of 8 to 20layers of alternating high and low index material, each layer having anoptical thickness of

/4 of

_(C4)=(750 nm/4)=187.5 nm.

FIG. 3 is a plot of reflectance vs wavelength for a LED device 11 with asilicon substrate 12, but without any sealant material 16, i.e. emittinginto the air, at a zero degree angle of incidence. The four overlappingreflectance bandwidths for the four aforementioned sections of thedielectric stack 17 defined by the center wavelengths

_(C1),

_(C2),

_(C3), and

_(C4) (not shown) are illustrated. FIG. 4 is also a plot of reflectancevs wavelength, illustrating the combined broadband reflectance bandwidthfor the combined dielectric stack 17, including the plurality ofsections as hereinbefore described. Accordingly, the dielectric stack 17provides 95% to 100% reflectance for a broadband spectrum of light (over300 nm) emitted into air at a zero degree angle of incidence.

In an alternative embodiment of the present invention, the dielectricstack 17 can be formed from twenty to fifty different sections, e.g.only one or two pairs of alternating high and low index layers, eachpair adapted to reflect a relatively narrow bandwidth, e.g. 10 nm to 25nm, defined by a centerwavelength. Accordingly, the dielectric stack 17could include quarter wavelength pairs for reflectance bandwidthscentered at 350 nm, 360 nm, 370 nm, . . . 820 nm, 830 nm, and 840 nm orsome smaller subset thereof. The optical thickness of each layer wouldbe the center wavelength divided by four (

_(C)/4). FIG. 5 is a plot of wavelength vs reflectance, and illustratesthe combined reflectance bandwidth for the aforementioned example for anLED device 11 with a silicon substrate 12, without sealant material 16at a zero degree angle of incidence. The second embodiment provides from98% to 100% reflectivity for a broadband spectrum of light (over 300 nmfrom 400 nm to 700 nm) emitted into air at a zero angle of incidence.

The aforementioned designs are typically optimized using thin filmdesign software. Examples of commercially available thin film designsoftware are ‘Escential Mcleod’, Filmstare, TfCalc, Optilayer. Thenumber of layers needed depends on the difference in refractive indicesbetween the high-index and low-index material. The larger the differencethe fewer layers are required.

The total height of the dielectric stack 17 can be in the range of 2 μmto 20 μm, preferably 2.5 μm to 15 μm, and most preferably between 7 μmand 12 μm, providing over 90% reflectivity, preferable over 92.5%reflectivity, for light from 400 nm to 680 nm at angles of incidencebetween 0° and 90°.

None of the aforementioned designs in FIGS. 4 to 6 a were optimizedusing the software as described above, i.e. the final design will not bea simple stack of layers with optical widths that are exactly a quarterwavelength of the center wavelength; however, the designs illustratedfrom FIG. 7 on have been ‘optimized’.

As illustrated in FIG. 6 a, the angle of incidence plays a factor in theoverall reflectivity of the dielectric stack 17. At several wavelengthsin the visible spectrum and at various angles of incidence thereflectivity is reduced by 5%, and at a few wavelengths and angles ofincidence the reflectivity is reduced by 10% to 15% or more for theaforementioned example of an LED device 11 with a silicon substrate 12,without sealant material 16. The spikes can be removed by optimization,as illustrated in FIG. 6 b, in which the reflectivity reductions atlarge and small angles of incidence are greatly reduced, providing over95% reflectivity at all angles and wavelengths between 400 nm and 675nm.

When the sealant material 16, with an index of refraction, e.g. 1.5,higher than air (1.0), is included in the LED device 11, there is adistinct reduction in reflectivity at certain angles of incidence, e.g.45° to 65°, as illustrated in FIG. 7, which is a plot of the angle ofincidence (AOI) vs reflectivity. The main reason for the drasticdecrease in reflectivity is an increase in the splitting of thepolarized s and p components of the light at the aforementioned angles.At a certain angle, i.e. the Brewster angle, while the s polarized lightis reflected all of the p polarized light is being transmitted, and inthe case of the LED device 11, lost due to absorption of the substrate12 or transmitted out the backside of the LED device 11.

With reference to FIG. 8, as a result of the Brewster phenomenon, theaverage reflection of the dielectric stack 17 in accordance with eitherof the aforementioned embodiments, across the visible spectrum (400 nmto 650 nm) and over an angle of incidence range of 0° to 90°, is reducedto approximately 93%, as compared to 97.9% for silver and 91.9% foraluminum.

A solution to the aforementioned problem, without sacrificingdurability, is illustrated in FIG. 9, in which an LED device 21, havingall of the same elements as the LED device 11, also includes a substrate22 with a durable metallic reflector layer 23, e.g. comprised ofaluminum, between the dielectric stack 17 and the conventional substratematerial 24. The metallic reflective layer 23 can be as thin as 10 nm to500 nm, but for practical purposes the reflective layer 23 is typicallybetween 35 nm and 120 nm and ideally between 50 nm and 70 nm, dependingupon the thickness requirements of the LED device 21.

With reference to FIG. 10, an LED device 31 is illustrated, in which theentire substrate 32 is comprised of a reflective metallic material,ideally aluminum, to provide the additional reflectivity required, aswell as to provide increased heat dissipation. A material having athermal conductivity of greater than 100 W/mK is preferred, while amaterial with a thermal conductivity of greater than 200 W/mK, e.g. Alk=250 W/mK, is more preferred. All the other elements, i.e. LED 13,electronic circuitry 14, sealing material 16, and dielectric stack 17are the same as in FIGS. 2 and 9. FIG. 11 illustrates how the lowestreflectivity for the combined dielectric stack/metallic reflector isover 93%, and FIG. 12 illustrates that the average reflectivity of thecombined reflector is over 98.5% over the entire visible spectrum, whichis greater than either a silver reflector or the dielectric stack 17.

Another advantage to the combined reflector is that the thickness of thedielectric stack 17 can be substantially reduced to between 3 μm and 6μm, by reducing the number of layers per section, without sacrificingtoo much performance in reflectivity. The total thickness of thedielectric stack 17 can be in the range of 2 μm to 20 μm, preferably 2.5μm to 12 μm, and most preferably between 3 μm and 8 μm, providing over90% reflectivity, preferably over 95% reflectivity, and most preferablyover 97% reflectivity for light having a bandwidth over 250 nm, e.g.from 400 nm to 680 nm, at angles of incidence between 0° and 90°.

Reducing the thickness of the dielectric stack 17 can be important whenlow thermal resistance through the dielectric stack 17 is required toincrease heat dissipation away from the LED 13. The table below providesa comparison of average reflectivities for reflector stacks 17 withdifferent thicknesses. The average reflectivity was calculated over anangle range of 0°-85° and a wavelength range of 400 nm to 680 nm.

Mirror Thickness (um) Average Reflectivity (%) 7.60 98.5 4.22 98.0 3.7097.6 3.11 97.4 Silver 97.8

With reference to FIGS. 13 a and 13 b, an issue arises with the standardembodiment, whereby reflection decreases substantially at high angles ofincidence, e.g. >45° for certain wavelengths, e.g. 600 nm to 750 nm,caused by identifying an angle shift with incidence angle that can bedescribed as a ‘red cliff’.

In general, the above-identified issue with the dielectric stack 17 canbe easily addressed by increasing the thickness, effectively ‘redshifting’ the angular dependence of the coating to longer, lessimportant wavelengths for particular applications. However, based onthermal considerations, the thickness of the dielectric stack 17 shouldnot be increased, as the LED 13 mount on top of the multi-layerdielectric stack 17 needs to dissipate heat therethrough to thesubstrate, e.g. 4, 12 or 32. During the development effort, much thickerdielectric stacks 17 provided superior optical performance, but sufferedfrom poor thermal transport across the thin film, effectively makingthem useless in this application.

With reference to FIG. 14, a key feature of an alternative embodiment ofan LED device 41 of the present invention is to split up the dielectricstack into at least two areas or segments: a first segment 47 asurrounding the LED 13 and electronic circuitry 14 with a first, thickerthickness, e.g. greater than 3.5 um, preferably approximately 4 um ormore for electrical isolation reasons and the remaining opticalperformance needed for the omnidirectional mirror comprising theremaining coating. Overall coating thickness can be up to 13 um, limitedby thermal performance of the structure. The thickness on the overallcoating could even be thicker than 13 um, but it would be costprohibitive.

A second segment 47 b, below the LED 13, is patterned withphotolithography or other suitable process, to have a second, thinnerthickness to allow the LED 13 to sit a suitable distance from thesubstrate 12/22/32 so that the electrical isolation is maintained andthe thermal impact of the additional thickness of the dielectric stackis minimized, e.g. between 1 um and 3 um, preferably about 2 um.

As typical LEDs are approximately 160 um tall (triangular in shape) andimmersed in sealant material 16, therefore the amount of the diode thatis obscured by the first, thicker, area 47 a is typically less than 2%(2+ um/160 um).

We claim:
 1. A light emitting diode (LED) device comprising: asubstrate; an LED for emitting light at a first wavelength; electricalcircuitry for providing power to the LED from external sources; awavelength conversion material covering the LED for converting lightemitted at the first wavelength to light of at least a secondwavelength, which combined with the light of the first wavelength formsa broadband light source; and a multi-layer dielectric structure ofalternating high and low index material layers in between the substrateand the LED providing both electrical insulation for the electricalconnections and reflectivity for the broadband light source; wherein themulti-layer dielectric structure has a first area below the LED with afirst thickness providing electrical isolation and heat transfer to thesubstrate, and a second area spaced apart from the LED with a secondthickness thicker than the first thickness surrounding the LED providingelectrical isolation and reflectivity for the light; wherein the firstand second thickness are in perpendicular direction from the substrate.2. The device according to claim 1, wherein the multi-layer dielectricstructure has a thickness of from 2.5 μm to 15 μm.
 3. The deviceaccording to claim 1, wherein the first area of the dielectric structurehas a thickness between 1 μm and 3 μm.
 4. The device according to claim3, wherein the second area of the dielectric structure has a thicknessbetween 3.5 μm and 13 μm.
 5. The device according to claim 1, whereinthe multi-layer dielectric structure is a broadband reflector reflectingat least 90% of light over a range of 400 nm to 680 nm.
 6. The deviceaccording to claim 1, wherein the first and second wavelengths combineto form white light.
 7. The device according to claim 1, wherein thewavelength conversion material comprises a plurality of differentmaterials for converting the light emitted at the first wavelength tolight at second and third wavelengths, which combine with the firstwavelength to form white light.
 8. The device according to claim 1,wherein the dielectric structure comprises a plurality of sections, eachsection for reflecting a different bandwidth, defined by a centerwavelength, of from 75 nm to 125 nm each, each section comprising aplurality of alternating high and low index layers, each layer having anoptical width approximately a quarter of the center wavelength.
 9. Thedevice according to claim 1, wherein the dielectric structure comprisesa plurality of pairs of high and low index material layers, each pairfor reflecting a different bandwidth of from 5 nm to 15 nm, defined by acenter wavelength, each layer having an optical width approximately aquarter of the center wavelength.
 10. The device according to claim 1,further comprising a sealant material in which the wavelength conversionmaterial is dispersed; wherein the sealant material is transparent tothe first and second wavelengths of light.
 11. The device according toclaim 10, wherein the transparent sealant material comprises one or morematerials selected from the group consisting of silicone, epoxy, glass,plastic, and polymer.
 12. The device according to claim 1, wherein thesubstrate comprises a layer of reflective metallic material; wherein thelayer of reflective metallic material and the multilayer dielectricstructure provide over 95% reflectivity for light at wavelengths between400 nm and 680 nm.
 13. The device according to claim 12, wherein thereflective metallic material has a coefficient of thermal conductivitygreater than 200 W/mK.
 14. The device according to claim 13, wherein thesubstrate comprises aluminum.
 15. The device according to claim 1,wherein the substrate consists of a reflective metallic material havinga coefficient of thermal conductivity greater than 200 W/mK.